Cavity induced stimulation of coal degasification wells using foam

ABSTRACT

An improved method for stimulating coal degasification wells comprising the injection of a foam into a coal seam which contains natural or induced fractures. Thereafter, high pressure gas is injected into the coal seam and suddenly released to cause disintegration of coal surrounding the borehole.

BACKGROUND OF THE INVENTION

The present invention relates to the production of gas from a coal seamand more particularly to an improved cavitation process wherein a foam,i.e. a viscous and compressible fluid, is injected into a coal seamfollowed by injection of high pressure gas which is then released toform a cavity in a coal seam.

Many subterranean coal seams have large volumes of hydrocarbon gases,usually including methane, trapped therein. These gases represent avaluable resource if they can be produced economically. Where a coalseam is to be mined later, it is beneficial from a safety standpoint toproduce as much of these gases as possible before commencement of miningoperations.

Presently, methane and any other gases are produced from the coalreservoirs through wells which are drilled into the coal seam. Once awell is drilled and completed, it is common to treat the coal seam inorder to stimulate the production of methane therefrom. Generally, thisinvolves some method of improving permeability of the coal seam. Onesuch commonly used stimulation treatment involves hydraulicallyfracturing the coal seam generally in the same manner as used withconventional oil and gas bearing formations, see for example, U.S. Pat.No. 4,995,463.

Another technique which has been proposed for stimulating a coal seam issometimes generally referred to as "cavity induced stimulation". In thistechnique, a wellbore is drilled through a coal seam and by use ofvarious techniques a cavity is formed within the seam adjacent thewellbore. As the cavity is formed, the vertical stress component whichnormally acts on the coal above the cavity is partially transferred tothe sides of the cavity which, in turn, causes the coal to become loadedinwardly as the cavity is being formed. This increased load wouldnormally be greater than the natural load bearing capability of the coaland the coal will fail and break up into small fragments. As the coalfragments are removed from the cavity through the wellbore, a largecavity is formed, thereby providing a relaxed zone into which existingfractures can open making the coal and surrounding rock more permeableto gas flow. This technique can be repeated until the bearing capacityof the coal equals or exceeds the redistributed stress. The net effectof forming a cavity into which surrounding coal can collect is theproduction of a highly permeable zone filled with fine grain coalparticles. For a more complete description of the mechanics involved ina typical cavity induced stimulation process, see "Cavity Stress ReliefMethod to Stimulate Demethanation Boreholes" A. K. Alain and G. M.Denes, SPE/DOE/GRI 12843, presented at the 1984 SPE/DOE/GRIUnconventional Gas Recovery Symposium, Pittsburgh, Pa., May 13-15, 1984.The cavity used in the above-described technique can be formed indifferent ways. For example, in the above-cited paper, the cavity in thecoal seam is disclosed as being formed by jetting water from the lowerend of a dual drill-type string while using compressed air to remove theresulting coal fragments.

Another known technique which has been used to form a cavity in a cavityinduced stimulation method involves the use of compressed air, nitrogenor other available gases. A wellbore is drilled and completed into acoal seam. A tubing string is then lowered into the wellbore and thewell annulus is closed. Compressed gas is supplied through the tubingstring to build up a high pressure in the coal seam adjacent thewellbore. The wellbore is then opened to suddenly vent the pressure,thereby allowing the gas within the cleats or fractures of the coal seamto expand and produce a back pressure which overcomes the induced hoopstress within the coal. Under proper conditions, the result of thesudden release of gas is that the coal fails and breaks into fragmentswhich are then removed from the tubing string. This process can berepeated until the desired permeable zone is formed within the seam.

While this gas cavitation process has increased the initial methaneproduction in some wells by as much as four to five fold, when comparedto wells which were hydraulically fractured, it has also been shown thatthis stimulation technique has not worked in other wells. As taught inU.S. Pat. No. 5,199,766 this failure may be due to the cleat densitybeing much less than it was in the successfully completed wells andlarge hoop stresses induced in the coal during the drilling process. Thelower cleat density increases the strength of the coal sufficiently thatthese hoop stresses cannot be overcome with the normal gas cavitationcompletion techniques. According to that patent, a solvent such asammonia is injected into the coal seam and allowed to dissolve materialsfrom the cleat structure for a period of time sufficient to weaken thatstructure. After the cleat structure is thus weakened, the cavityinduced stimulation technique has been found to effectively stimulatethe coal degasification.

In two other situations the gas cavitation process has also been foundto be unsuccessful. The process was not effective in wells which hadbeen previously hydraulically fractured as discussed above. In general,the process has not been considered applicable to previouslyhydraulically fractured wells, because such wells are cased through thecoal seam with the fracturing process, and gas production, occurringthrough perforations formed in the casing adjacent the coal seam.Similar results occurred where the formation contained highly conductivenatural fractures.

SUMMARY OF THE INVENTION

The present invention provides a cavity induced stimulation method forimproving the production of fluids such as methane from a subterraneancoal formation or seam. In carrying out the method, a well is drilled toa point adjacent a coal seam and is cased to that point. The wellbore isthen extended beyond the cased wellbore and into the seam. Acompressible and viscous fluid, i.e. foam, is then pumped into the coalseam to a depth corresponding to the desired cavity size. The foam maybe displaced into the seam by use of compressed gas. A gas such as airor nitrogen is pumped at high pressure down the wellbore and into thecoal seam as in a conventional gas cavitation process. When anappropriate gas pressure is established in the formation surrounding thewellbore, the gas pressure is suddenly released to allow the pressurizedgas to flow back from the formation and break the coal into fragmentswhich then can be removed through the wellbore. The process may berepeated as appropriate to increase the cavity size, if desired.

The present invention has also proved useful as a recompletion techniquefor wells which were originally completed by casing through the coalseam and hydraulically fracturing the coal seam through perforationsformed in the casing adjacent the coal seam. A window in the casing maybe milled to allow drilling of a sidetrack open borehole through thecoal seam so that the above described process may be performed. The sameprocess may also be performed through the perforations to form a cavityin the coal seam around the perforated casing.

BRIEF DESCRIPTION OF THE DRAWING

The present invention may be better understood by reading the followingdetailed description of the preferred embodiments with reference to theaccompanying drawing which is an elevational view, partly in crosssection, of a subterranean coal seam or formation with a wellborecompleted therein for practice of the foam enhanced cavity inducedstimulation method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As noted above, the gas cavitation process has proven to be lesseffective, or completely ineffective, in coal seams which containnatural or artificial fractures. We believe that the fractures interferewith the process both during injection of gas and during thedepressurizing step. During gas injection the fractures divert the gasfrom the coal matrix into the fractures, away from the borehole and makeit more difficult to establish the desired high pressure conditions.During the depressurizing step, the fractures again divert the gasaround the coal matrix and allow a rapid loss of pressure withoutgenerating a desired backstress on the coal which is required to causefailure of the coal seam. We have overcome these problems by injectionof a compressible fluid, i.e. a foam, into the coal seam prior toinjection of high pressure gas in the cavity induced stimulationprocess. Inclusion of a gelling agent in the foam should furtherincrease its effectiveness primarily by stabilizing the foam, but thishas not proven necessary in initial testing. The foam preferentiallyflows into the fractures, reducing their effective permeability andthereby diverts gas injected during the gas cavitation process into thecoal matrix and allows the desired establishment of a high gas pressurezone near the wellbore. Likewise, during the depressurizing step, thefoam prevents the rapid loss of the gas through the fractures and allowsit to apply the proper forces to the coal matrix as desired.

With reference to the FIGURE, there is illustrated a wellbore 10 whichhas been drilled to and completed in a coal seam 11. Preferably, thewell is first drilled through the overlying earth formations 22 to thetop of coal seam 11. Surface casing 12 is then installed and sealed inplace by cement 13. The lower open portion of the borehole 24 is thencompleted through coal seam 11. A tubing 14 is installed to provide ameans for circulating fluids from the lower end of the borehole. A valve16 and conduit 20 are provided in communication with the annulus 26between tubing 14 and casing 12. For example, air or other fluids may beflowed down tubing 14 and returned through annulus 26 to remove anymaterials remaining in the open borehole section 24 before thestimulation process is commenced. It is also preferred in the presentinvention that all liquids in the lower open hole section 24 bedisplaced with air.

After thus cleaning out the borehole 10, a foam forming fluid is pumpeddown tubing 14 to the open hole section 24 of borehole 10. The foamforming fluid is pumped into the coal seam at matrix rates, that isbelow the minimum in situ stress. Compressed air or nitrogen is thenpumped down tubing 14 to displace the foam forming fluid from theborehole into the coal seam 11. As the fluid is displaced by the gas,the foam is formed in situ. The quantities and types of materials areselected based on type of coal and the desired depth of penetration offoam, typically from about-five to about eight feet from the borehole.In a typical case, this would require about 12 to 15 barrels of foamforming liquid for a 25 foot thick coal seam with 5% porosity.

Foams which are believed suitable for this purpose include a foamcomposed of water, surfactant and one or more of air, nitrogen and CO₂.Suitable surfactants include coco-trimethyl quaternary amines,perflourinated quaternary ammonium iodide, nonylphenol +10 mols ofethylene oxide. The foam may be stabilized by addition of a gellingagent including guar, guar derivatives, cellulose, cellulosederivatives, xanthan, and xanthan derivatives. These materials arebelieved to be useful at ambient conditions, that is, they do notrequire application of additional heat or extreme pressures.

A foam of 70 to 80 quality is preferred for this application. For thepurposes of this process this means a foam which is 70% to 80% gas byvolume and 20% to 30% liquid at downhole conditions. A typical foamforming fluid would be made by mixing 0.2% to 0.5% by weight ofnonylphenol +15 mols of ethylene oxide with water, or two to fivegallons per thousand gallons of water. In a typical well, ten barrels ofthis foam forming fluid would be injected from the borehole into thecoal seam and then displaced with 500 to 600 standard cubic feet of gasper barrel of fluid to form the foam in situ. The optimum quantity,concentration and viscosity of the foam will depend on seam thicknessand coal type and properties.

After the foam is injected, a gas such as air or nitrogen is pumped downtubing 14 at high pressure, but below formation fracture pressure, andinto the coal seam 11. Note that this gas injection step can be simply acontinuation of the gas injection which forms the foam in place. As in anormal gas cavitation process, pumping is continued until a sufficientbottom hole pressure is achieved and high pressure gas has penetratedsufficiently far into coal seam 11. Valve 16 is then opened to allowhigh pressure gas to be released from the wellbore 10 through conduit 20to suddenly drop the wellbore pressure. The foam treatment reduces thediversion and loss of injected gas in the coal seam 11 during theinjection process allowing the desired pressure buildup. The foam alsoprevents a rapid bleedoff of the pressurizing gas through highlypermeable fractures during the depressurizing step to thereby prolongthe tensile action on the coal. As a result the gas flowing back out ofseam 11 will cause the desired cavitation about the borehole 10. Thecoal particles generated by the process may then be removed from theborehole by circulation as done at the beginning of the process.

The present invention was developed and initially tested to recompletewells which were originally completed and stimulated by hydraulicallyfracturing. These wells differ from the preferred completion techniquein that they are normally cased through the coal seam. The casing isperforated adjacent the coal seam to provide communication between theborehole and the coal seam for the injection of hydraulic fracturingmaterials and for production of gas. In the initial tests of the presentinvention the casing was removed partly in the coal seam by aconventional milling process to provide an opening or window fordrilling a sidetrack well from the original wellbore. After thus openingthe casing and drilling a sidetrack in the coal seam, the well isfunctionally the same as the preferred embodiment shown in the FIGURE.That is, the well is cased to the top of, but not through, the coal seamand the wellbore continues through the coal seam as an open holecompletion. The open hole portion of the well is normally in the portionof the coal seam which was fractured by the previous hydraulicfracturing completion process. After this sidetracking step, the processdescribed above was carried out in the well. In the first test well,flow rate was improved from 930 MCFPD to 1.5 MMCFPD, that is, it wasincreased by a factor of 1.6 as compared to the production from the samewell after hydraulic fracture stimulation. In the second test well, flowrate was improved from 800 MCFPD to 1.4 MMCFPD, that is, it wasincreased by a factor of 1.75 as compared to the production from thesame well after hydraulic fracture stimulation.

In the initial testing we used as a foam forming fluid a blend ofmaterials identified by the contractor, Halliburton Well Services, as1/3 "SSO-21" and 2/3 "AQF II". The description of these materials, asgiven by the contractor is: "SSO-21" is a microemulsion of 71% (50%ethylene glycol monobutyl ether +50% of a C₈ Alcohol reacted with 2moles of ethylene oxide) +14.5 % (50% active nonyl phenol alkyloxylatedwith 20 moles of ethylene oxide) +14.5 % (2-ethyl hexanol); and "AQF II"is generically described as a alkyl olefin (C₁₄ to C₁₆) sulfonate.

After the initial testing, we have found that the relatively expensivemilling and sidetracking process is not necessary for effectiverecompletion of wells previously completed and stimulated byconventional hydraulic fracturing techniques. The process of the presentinvention can effectively be performed through the original perforationsin the casing. We believe that it is desirable to add additionalperforations in the casing adjacent the coal seam and have done so intesting the present invention. We have successfully recompleted suchwells by adding perforations and then applying the foam enhanced cavityinduced stimulation process described above through the perforations.The process effectively breaks the coal into particles small enough tobe produced through the perforations so that a cavity is formed aroundthe borehole outside the casing. Initial testing indicates formation ofgood cavity completion and improved flow, but actual flow ratemeasurements are not available.

As an alternative to sidetracking the wellbore, or recompleting throughthe perforations, it would be possible to remove all of the originalcasing in the coal seam by milling. However this would be the mostexpensive and most difficult way of reaching the well configurationshown in the FIGURE and is not considered to be practical on economicgrounds.

As illustrated by the dash lines a, b, c and d in the FIGURE, theinitial cavitation process may typically generate a cavity along linesa. Repeated steps can expand the cavity to the positions b, c and d, asdesired.

While the present invention has been illustrated and described withreference to particular apparatus and methods of operation, it isapparent that various changes can be made therein within the scope ofthe present invention as defined by the appended claims.

What is claimed is:
 1. A cavity induced stimulation method of the typein which injection and release of high pressure gas from a subterraneancoal seam adjacent a wellbore is used to form a cavity adjacent thewellbore for improving the production of fluids from the subterraneancoal seam comprising:(a) completing a wellbore into said coal seam; (b)flowing a foam forming fluid down the wellbore to said coal seam andinto said coal seam; (c) flowing a gas down the wellbore and into saidcoal seam at high pressure to generate foam in situ and to generate anarea of high pressure gas in the coal seam adjacent the wellbore; and,(d) releasing the pressure in said wellbore to generate a cavity in thecoal seam adjacent the wellbore.
 2. The method of claim 1, wherein saidfoam is displaced into said coal seam to a depth of from about five toabout eight feet from said wellbore.
 3. The method of claim 1, whereinsaid foam is a mixture of water and surfactant and at least one gasselected from the group comprising air, nitrogen and CO2.
 4. The methodof claim 3, wherein said foam also includes a gelling agent.
 5. Themethod of claim 4, wherein said gelling agent is selected from the groupcomprising guar, cellulose and xanthan and derivatives of guar,cellulose and xanthan.
 6. The method of claim 1, furtherincluding:repeating steps (b), (c) and (d) of claim 1 one or more times.7. A cavity induced stimulation method of the type in which injectionand release of high pressure gas from a subterranean coal seam adjacenta wellbore is used to form a cavity adjacent the wellbore for improvingthe production of fluids from an existing well which has been completedin the subterranean coal seam and used to hydraulically stimulate thecoal seam comprising:(a) flowing a foam forming fluid down the wellboreto said coal seam and into said coal seam; (b) flowing a gas down thewellbore and into said coal seam at high pressure to generate foam insitu and to generate an area of high pressure gas in the coal seamadjacent the wellbore; and, (c) releasing the pressure in said wellboreto generate a cavity in the coal seam adjacent the wellbore.
 8. Themethod of claim 7, wherein said foam is displaced into said coal seam toa depth of from about five to about eight feet from said wellbore. 9.The method of claim 7, wherein said foam is a mixture of water andsurfactant and at least one gas selected from the group comprising air,nitrogen and CO2.
 10. The method of claim 9, wherein said foam alsoincludes a gelling agent.
 11. The method of claim 10, wherein saidgelling agent is selected from the group comprising guar, cellulose andxanthan and derivatives of guar, cellulose and xanthan.
 12. The methodof claim 7, further including:repeating steps (b), (c) and (d) of claim1 one or more times.
 13. The method of claim 7, wherein the existingwell as originally completed is cased through the coal seam and thecasing is perforated adjacent the coal seam, and wherein the method ofclaim 7 is performed by flowing fluids into and producing fluids fromthe coal seam through the perforations.
 14. The method of claim 13,wherein prior to performing steps (a), (b) and (c), additionalperforations are formed in the casing adjacent the coal seam.
 15. Themethod of claim 7, wherein the existing well as originally completed iscased through the coal seam and wherein prior to performing steps (a),(b) and (c), at least a portion of the casing is removed adjacent thecoal seam and a sidetrack borehole is drilled through said coal seam.